(1) Field of the Invention
This invention relates to a semiconductor optical amplification module, an optical matrix switching device, and a drive circuit and, more particularly, to a semiconductor optical amplification module for controlling the passing through of an optical signal, an optical matrix switching device for switching an optical output path, and a drive circuit for driving such a semiconductor optical amplification module.
(2) Description of the Related Art
High-speed large-capacity optical communication devices are required in optical communication networks to build multimedia networks in the future. Optical packet switching systems using high-speed optical switches which operate in several nanoseconds (ns) are being researched and developed as systems which realize high-speed large-capacity optical communication. Semiconductor optical amplifiers (SOA) are devices which can perform switching in several nanoseconds, that is to say, which can perform high-speed switching. It is expected that SOAs are applied to, for example, optical matrix switches in optical packet switching systems.
FIGS. 16A, 16B, and 16C are views for describing the operation of an SOA. In FIG. 16 A, an SOA 100 and a drive circuit 101 for driving the SOA 100 are shown. In addition, optical signals (optical packets) inputted to the SOA 100 are shown in FIG. 16A. Driving current supplied from the drive circuit 101 to the SOA 100 is shown in FIG. 16B. Optical signals outputted from the SOA 100 are shown in FIG. 16C.
A control signal is inputted to the drive circuit 101. The drive circuit 101 outputs the driving current shown in FIG. 16B to the SOA 100 according to the control signal.
When the driving current outputted from the drive circuit 101 is injected into an optical signal amplification area, the SOA 100 amplifies the optical signals propagating through the optical signal amplification area. By turning on/off the driving current, as shown in FIG. 16B, the SOA 100 is used as a gate element for the optical signals.
It is assumed that the driving current the timing of which is shown in FIG. 16B is supplied. The SOA 100 amplifies and outputs the optical signals #1 and #3 of the optical signals #1 through #3 shown in FIG. 16A. The SOA 100 attenuates and outputs the optical signal #2. As a result, the optical signals shown in FIG. 16C are outputted from the SOA 100.
FIG. 17 is a view showing an example of the structure of the drive circuit shown in FIG. 16A. As shown in FIG. 17, the drive circuit includes resistors R101 through R104, an operational amplifier (op-amp) OP101, and power sources P101 and P102. In addition to the drive circuit, an SOA module 110 including the SOA 100 is shown in FIG. 17. In FIG. 17, a parasitic inductance component produced by a substrate pattern of the drive circuit is equivalently shown as an inductor L101 and a parasitic inductance component produced by wirings in the SOA module 110 is equivalently shown as an inductor L102.
The op-amp OP101 shown in FIG. 17 is included in a non-inverting amplifier. The output current capacity of the op-amp OP101 is 300 mA or more. The settling time of the op-amp OP101 is about 2 ns. That is to say, the op-amp OP101 is a high-speed op-amp. A square-wave signal for turning on/off the SOA 100 is inputted to a non-inverting input terminal of the op-amp OP101. In FIG. 17, a square-wave signal is generated by a signal generator SG and is inputted. However, a desired square-wave signal is actually supplied from a field programmable gate array (FPGA) or a logic buffer circuit according to a control signal sent from a control section.
FIG. 18 is a view for describing ringing of the drive circuit shown in FIG. 17. A waveform VSG shown in FIG. 18 indicates voltage at a point VSG shown in FIG. 17. A waveform VSOA shown in FIG. 18 indicates voltage at a point VSOA shown in FIG. 17. A waveform ISOA shown in FIG. 18 indicates an electric current at a point ISOA shown in FIG. 17. Each waveform shown in FIG. 18 indicates a result obtained by doing a simulation by the use of element values shown in FIG. 17.
Setting is performed so that the voltage at the point VSG shown in FIG. 17 will be 1.5 V at the time of the signal generator SG being in an on state and so that the voltage at the point VSG shown in FIG. 17 will be 0 V at the time of the signal generator SG being in an off state. The non-inverting amplifier including the op-amp OP101 is set so as to output voltage which is equal to input voltage.
When a driving current of about 300 mA is passed through the SOA 100, an optical amplification factor of about 10 dB can be obtained. When driving current is decreased, the SOA 100 exhibits an optical attenuation characteristic. In this example, the operation of the SOA 100 is as follows. When a voltage of 1.5 V is outputted from the non-inverting amplifier, a driving current of about 300 mA runs through the SOA 100 and the SOA 100 turns on. When a voltage of 0 V is outputted from the non-inverting amplifier, an electric current does not run through the SOA 100 and the SOA 100 turns off.
Optical leakage may occur even when the SOA 100 is in anoff state. In this case, crosstalk may occur in a multiplexing coupler to which output from the SOA 100 is sent. Driving voltage of the SOA 100 must be set to 0.65 V or less in order to obtain the off state of the SOA under which crosstalk does not occur (in order to prevent the optical leakage of the SOA 100).
As described in FIG. 17, the drive circuit equivalently includes the inductor L101 and the SOA module 110 equivalently includes the inductor L102. In addition, the SOA 100 itself has a junction capacitance of 40 to 70 pF. Accordingly, when the SOA 100 is switched from the on state to the off state, electric charges charged at on time discharge and large ringing occurs in the drive circuit because of back electromotive force generated by the inductors L101 and L102.
As shown by the waveform VSOA of FIG. 18, ringing occurs in the circuit shown in FIG. 17 and a voltage of 0.65 V or more is generated (arrow A101). Therefore, though the drive circuit outputs driving current so as to put the SOA 100 in the off state, the SOA 100 reaches a light emission level. As a result, high-speed optical switching is inhibited.
FIG. 19 is a view showing an example of the structure of a drive circuit which suppresses ringing. Components in FIG. 19 that are the same as those shown in FIG. 17 are marked with the same symbols and descriptions of them will be omitted.
As shown in FIG. 19, a resistor R110 is connected between the drive circuit and an SOA module 110. In addition, an inductor L103 is connected to an output of an op-amp OP101. With the drive circuit shown in FIG. 19, the occurrence of ringing is suppressed by inserting the resistor R110.
FIG. 20 is a view for describing ringing of the drive circuit shown in FIG. 19. A waveform VSG shown in FIG. 20 indicates voltage at a point VSG shown in FIG. 19. A waveform VSOA shown in FIG. 20 indicates voltage at a point VSOA shown in FIG. 19. A waveform ISOA shown in FIG. 20 indicates an electric current at a point ISOA shown in FIG. 19. Each waveform shown in FIG. 20 indicates a result obtained by doing a simulation by the use of element values shown in FIG. 19.
As described in FIG. 19, the occurrence of ringing is suppressed by connecting the resistor R110 to output of the drive circuit. However, voltage drops by inserting the resistor R110. As a result, a large driving voltage of 10.5 V must be applied to pass a driving current of 300 mA through the SOA 100 and to turn on the SOA 100. Accordingly, the power consumption of the drive circuit increases.
For example, the power consumption of the resistor R110 reaches 2.7 W at on time (300 mA). Moreover, the power consumption is high, so a resistor of large size must be used as the resistor R110. This leads to an increase in the size of the circuit. Furthermore, output voltage of the op-amp OP101 becomes higher, so it is difficult to use a high-speed op-amp the settling time of which is about 2 ns.
A laser drive circuit in which the rising of laser output is made sharp by passing an overshoot that otherwise flows to a laser diode through an LCR circuit is proposed (see, for example, Japanese Utility Model Laid-Open Publication No. Hei5-48369).
As stated above, if the resistor is connected to the output of the drive circuit to suppress the occurrence of ringing, power consumption increases and the size of the circuit increases. Furthermore, high-speed optical switching is inhibited.